Purification of methanol



PnninrcA'rroN on ivmrnANoL No Drawing. Application January 2, 1957Serial No. 632,013

3 Ciainis. (Cl. 260-643) This invention relates to the purification ofsynthetic methanol and, more particularly, it pertains to a method forincreasing the so-called permanganate time of purified methanol.

In order to meet the rigid specifications prescribed for pure methanol,it is necessary that all traces of impurities be removed from thealcohol. It is known that methanol produced synthetically byhydrogenation of carbon monoxide possesses certain oxidizablecontaminants the last traces of which are substantially impossible toremove even by means of highly efiicient fractionating columns. Thepresence of these oxidizable impurities such as carbonyl impurities ofan aldehydic or ketonic nature, for example, under ordinary conditionsgive rise to objectionable odor and impart other undesirable propertiesas well to the methanol which prevent it from meeting specifications.Because of the obvious advantages of methanol which is free of theseoxidizable impurities, many methods have been proposed and developed fortreating methanol to secure the pure product. Among the more successfulprocesses thus developed and one in Wide commercial use is that whichconsists of the several steps of treating crude methanol with sulfuricacid and potassium permanganate, neutralizing the excess acid present,and then distilling the thus treated material to recover the puremethanol. While this procedure renders methanol generally suitable formost uses, We have now discovered that the permanganate time of methanolpurified by this method may be significantly lengthened if a specificand very small amount of an organic amine is added to the methanoldistillate.

It is an object of the present invention, therefore, to provide animproved process for the purification of methanol.

It is a further object of the invention to provide a method forincreasing the permanganate time of methanol containing contaminantsoxidizable by permanganate.

Other objects and advantages of the invention will become obvious fromthe following description of the invention.

According to the invention, contaminated or crude methanol containingoxidizable impurities is treated with sulfuric acid and potassiumpermanganate, excess sulfuric acid is then neutralized by the additionof an alkali metal hydroxide, the resulting mixture is distilled toseparate methanol therefrom, and an amount of an organic amine, such astriethyl amine, for example, in the range from about 0.3 mole to about 3moles of the amine per million moles of distilled methanol is then addedto the distilled methanol.

The following example will serve to illustrate the invention but is notto be construed as limiting it in any manner except as it is limited inthe appended claims.

Example I Varying amounts of triethylamine were dissolved in samples ofrefined methanol and the effect of the Patent Q 2,846,477 Patented Aug.5, 1958 amine on the permanganate time of the methanol was observed.Refined methanol in this case was product or C. P. methanol producedfrom the crude alcohol synthesized by the catalytic reaction of hydrogenand carbon monoxide. The crude methanol was contacted with sulfuric acidand a substantially saturated aqueous solution of potassium permanganatein a suitable still or vessel provided with means for agitation.

A reaction time of approximately 20 minutes was provided. The treatedalcohol was then neutralized by the injection of sodium hydroxide with aexcess of caustic being employed. The mixture was filtered to remove anysolids and the methanol filtrate subjected to fractionation in a60-plate column. The refined product methanol was taken off as asidestream, while water, heavy alcohols and other high boilers wereremoved as bottoms from the still. Permanganate time was determined bytreating a 50-ml. sample of methanol with 2.0 ml. of 0.02% solution ofpotassium permanganate at 15 C. and measuring the time required forreduction of the permanganate as evidenced by disappearance of thepurple color determined by matching a color standard. Results of thetreatment of the refined methanol was tabulated below. These are indeedsurprising in view of the fact that the refined product is collected asthe distillate in a fractionation system wherein the still contents aredistinctly alkaline.

Permanganate The quantity of amine required in the method of theinvention is highly critical. Quantities in excess of 5 moles of amineper million moles of methanol will result in a degredation in methanolquality as measured by permanganate time rather than an improvement.Preferred quantities for improving permanganate time are those in therange from about 0.3 to about 3 moles of amine and optimum improvementis effected with quantities of amine in the range from about 0.452.5moles per million moles of methanol.

Any organic amine may be employed as a treating agent in practicing themethod of the invention. Aliphatic amines, however, are preferred andprimary, secondary and tertiary alkyl'amines such as methyl amine, ethylamine, propyl amine, dimethyl amine, trimethyl amine, diethyl amine,tert-butyl amine, diisopropyl amine and the like are especiallysuitable. Particularly preferred as those alkyl amines containing from 1to 18 carbon atoms.

The permanganate treatment by which the crude methanol is refined toproduce the alcohol subjected to the treatment of the invention may beconducted under a wide variety of conditions. In addition to potassiumpermanganate, for example, sodium or other permanganates may be used.The methanol is generally treated under non-alkaline, or preferably,mildly acidic conditions. Acidity is easily regulated by the addition ofsmall amounts of a mineral acid such as sulfuric acid with the quantityof acid employed being kept about equal in weight percent to the amountof permanganate used. The quantity of permanganate used may range fromabout 0.02 to 3.0% by weight of the alcohol being treated depending uponthe source of the crude methanol and the temperatures at which theoxidation is being effected. More permanganate is required, for example,

at temperatures appreciably below atmospheric, particularly if morepractical reaction times are desired. Aqueous solutions of permanganateare ordinarily employed but the permanganate my likewise be added in theform of a finely ground solid if so desired. In this case, the timerequired for disappearance of the permanganate color is about twice aslong as that needed when using an aqueous solution. ordinary oratmospheric temperatures must be allowed for the oxidation reaction tobe completed. At higher temperatures, up to 5060 C. for instance,reaction is more rapid but a secondary reaction is accelerated whichresults in conversion of the methanol to formaldehyde. Preferably,therefore, the permanganate is contacted with the alcohol attemperatures from about 20 to about 35 C.

In the neutralization step, either solid, flake caustic, orcommercialalkali solutions may be used but the solution is preferred. Here, also,potassium or other alkali metal hydroxides are equally as suitable asthe caustic soda used in the examples. For satisfactory results and toinsure that all the acid is neutralized, a 100% excess of caustic isemployed so that the amount of caustic used lies within the rangefromabout 0.04 to about 6% by weight of the alcohol being treated.

Removal of solids from the neutralized alcohol is effected byfiltration, preferably using a leaf filter, although other conventionalfiltration techniques are also suitable. v i

The final stage in the preparation of the refined alcohol is a simplefractional distillation to recover the pure methanol either overhead orfrom a sidestream and any distillation column having fifty or moreplates may be operated in the conventional manner for this finalrectification.

Usually from 20 to 60 minutes at ni h What is claimed is:

1. In a process for the purification of crude synthetic methanol whichconsists of treating said methanol with a mineral acid and awater-soluble permanganate, neutralizing the acid present with an alkalimetal hydroxide, filtering to remove any solids, and distilling themethanol filtrate to produce a purified methanol distillate, theimprovement which comprises adding to said methanol distillate an amountof a tertiary aliphatic amine such that not more than 5 moles of amineare present per million moles of methanol distillate whereby thepermanganate time of said purified methanol is significantly lengthened.

2. In a process for the purification of crude synthetic methanol whichconsists of treating said methanol with sulfuric acid and awater-soluble permanganate, neutralizing the acid present with sodiumhydroxide, filtering to remove any solids, and distilling the methanolfiltrate to produce a purified methanol distillate, the improvementwhich comprises adding to said methanol distillate from about 0.45 toabout 2.5 moles of a tertiary aliphatic amine per million moles ofmethanol distillate whereby the permanganate time of said purifiedmethanol is significantly lengthened.

3. The process of claim 2 wherein said amine is triethyl amine.

References Cited in the file of this patent UNITED STATES PATENTS1,987,601 Burke Ian. 15, 1935 2,227,485 Bump Ian. 7, 1941 FOREIGNPATENTS 494,985 Great Britain Nov. 4, 1938

1. IN A PROCESS FOR THE PURIFICATION OF CRUDE SYNTHETIC METHANOL WHICHCONSISTS OF TREATING SAID METHANOL WITH A MINERAL ACID AND AWATER-SOLUBLE PERMANGANATE, NEUTRALIZING THE ACID PRESENT WITH AN ALKALIMETAL BY HYDROXIDE. FILTERING TO REMOVE AND SOLIDS, AND DISTILLING THEMETHANOL FILTRATE TO PRODUCE A PURIFIED METHANOL DISTILLATE, THEIMPROVEMENT WHICH COMPRISES ADDING TO SAID METHANOL DISTILLATE AN AMOUNTOF A TERTIARY ALIPHATIC AMINE SUCH THAT NOT MORE THAN 5 MOLES OF AMINEARE PRESENT PER MILLION MOLES THAN 5 MOLES OF AMINE ARE PRESENT PERMANGANATE TIME OF SAID PURIFIED METHANOL IS SIGNIFICANTLY LENGTHENED.